This invention relates to an improvement in an ultrafiltration process for purifying solutions of polymeric colorants.
U.S. Pat. No. 3,920,855 of Dawson et al, issued Nov. 18, 1975 and copending patent application, Ser. No. 520,530, filed Nov. 4, 1974 by Gless et al now U.S. Pat. No. 4,018,826, issued on Apr. 19, 1977 and Ser. No. 638,730, filed Dec. 8, 1975 by Wingard et al, and now U.S. Pat. No. 4,051,738, and Japanese Patent Applications of Tanabe Seiyaku such as 41-14433, 41-14434 and 44-13382 disclose that colors can offer advantages in polymeric form. U.S. patent application Ser. No. 727,097 filed on Sept. 27, 1976 by Ned Weinshenker, and now abandoned, further discloses that ultrafiltration is useful to purify polymeric colorants and free these colorants from undesired monomeric and low molecular weight polymeric impurities. The present invention concerns a way to improve the efficiency of this purification.